Question

Please answer question 2b. If you are writing by hand please make handwriting as readable as possible.

2. Lithography:
(a) Polymers containing aromatic rings are more likely to absorb UV photons, leading to chain scission. Would this make them negative or positive resists for lithography. (2 points)

(b) Explain briefly how lift off works. (2 points)

(c) Explain why chips are exposed with a stepper (so only a small field of view is illuminated at a time) in deep UV lithography? (2 points)

(d) Looking at the electron trajectories in Fig 5.8.explain why it would be useful to have a resist with a fairly sharp threshold exposure needed to make it soluble. (2 points)

(e) It is desired to etch away the whole thickness of a silicon wafer with a well- defined etch angle. How can this be achieved? ( 2 points)

Answer 1

The lift-off process in microstructuring technology is a method of creating structures (patterning) of a target material on the surface of a substrate (e.g. wafer) using a sacrificial material (e.g. photoresist). It is an additive technique as opposed to more traditional subtracting technique like etching. The scale of the structures can vary from the nanoscale up to the centimeter scale or further, but are typically of micrometric dimensions.

Basic principle :

In lift-off processing, the coating is applied to existing
photoresist structures.
The subsequent actual lift-off removes the resist
structures together with the material deposited
thereon, while the material applied directly to the
substrate through the openings of the resist mask
remains there as desired.

Advantages and disadvantages :

The lift-off procedure only then achieves reproducible defined structures when a coating of the resist
sidewalls is prevented which is impossible in isotropic sputter processes.
With some materials such as gold or silicon nitride, wet-chemical etching is problematic due to the poor
adhesion of the resist masks applied to it and therefore dry etching or lift-off is a reasonable alternative.
Wet-chemical etching processes are not applicable if the required chemicals cannot be used for, e.g. work
safety reasons.
If there is a high heating of the substrate due to the coating process and its duration, lift-off processes
are critical, because here the already existing photoresist structures are thermally affected (softening or
strong cross-linking).